[LLVMdev] Proposal: New IR instruction for casting between address spaces

[Nick Lewycky]

Chris Lattner wrote:
>
> On Sep 13, 2012, at 5:55 PM, Mon Ping Wang<monping at apple.com>  wrote:
>>>> The pointer size is target dependent so it seems strange to choose an arbitrary size to convert to and from. Are you making a practical argument that 64b is sufficient on all machines so all targets can use that?  In other words, pointers>  64 doesn't make any sense in terms of the address space? (A pointer to be>  64 if clients want to use some upper bits to track some state I guess).
>>>>
>>>> In terms of the three new instructions, one could argue that ptrtoint and intoptr has the same issue or those can also explode in a similar way.  To use them, this seems target dependent so unless we really want to support all the various addressing structures, I rather not have them.
>>>
>>> My point is that any producer of this sort of pointer cast is already necessarily target specific (it is generating target-specific address space numbers!).  If the front-end knows the address space to use, it can know a safe integer size to use.
>>>
>>
>> It depends on what the address space is used for. If I'm logically partitioning an address space that overlap my pointer size may all be the same size so this issue doesn't come up other than I know the pointer size are the same.
>
> Sure, in that case, use bitcast.
>
>> My understanding is that is becoming an issue since a pointer type size could be different for different address space.  I agree for the case where the pointer size is address space dependent that the client has to understand the size and the properties to decide if they need to do truncation, sign extension or zero extensions.
>
> Right.
>
>> This is a problem for auto upgrade as well.  Today, we have bit cast between same size pointers for different address space.  We would need to do something special for auto upgrade here since the proposal is to not allow bit cast between pointers of different address spaces.
>
> I haven't followed the details of the proposal, but I think it makes perfect sense to continue using bitcast for ptr/ptr casts within the same pointer size.

I don't want whether a module passes the verifier to depend on target 
data. I also don't want bitcasts that change width to pass the verifier. 
Hence the thinking was that bitcasts across address spaces would be 
rejected and replaced with something else (initially ptrtoint/inttoptr 
pairs were proposed, then a new instruction).

Nick

   If you do that, then there is no auto-upgrade issue: all existing bc 
files can just be assumed to have the same pointer size.
>
> -Chris
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